Prune process can have a large memory footprint in case of a large files list

When Artifactory contains a large number of artifacts (the size does not affect) and then activating the Prune process this can cause out of memory exception.  

Steps to reproduce:

1. Set up Artifactory with recommended java parameters.
2. fill Instance with artifacts that have different SH1.
3. Activate Prune process Admin tab->maintenence->Prune process

After some time Artifactory will be down with OutOfMemoryException

 It is possible to reproduce when increasing the JVM parameter but also the number of artifacts.